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PostPosted: Tue Sep 12, 2017 1:40 pm 
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Joined: Wed Jul 29, 2015 12:30 pm
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Year and Trim: 1997 Pontiac Bonneville SE
I could use some help from you guys regarding my 97 SLE and its strange fuel-related problems and strange fuel-pressure measurements.

First, I've had problems with engine speed oscillations (about once per second) while at about 60 mph and 2200 rpm during a long gradual incline. I've also had starting problems (needed to pump gas, then wait a minute, then try again) after a half-hour after I shut off the engine to do some shopping.

Yesterday, I decided to test my engine's Vacuum and Fuel Pressure. My vacuum seemed OK, but my fuel pressure was kinda strange. Here's what I found, along with some findings from my Haynes Repair Manual:

My vacuum specification is 17 - 22 in. of vacuum, at idle. I measure 18 inches of vacuum at my Fuel Pressure Regulator, using a "T" hose connection and a Sun engine vacuum gauge.

My Fuel pressure specification is 40-47psi, with ignition on but engine not cranking, and also while engine is running. My Haynes Repair Manual tell me that when fuel pressure is too low, it could be a blocked fuel filter, bad fuel pressure regulator, bad fuel pump, or some combination. My manual also says that when fuel pressure is too high, it could be that the fuel return line is blocked.

I measured the Fuel Pressure before the fuel filter, right at the tank's output, using a "T" hose connection and a Fuel Pressure Gauge kit I borrowed from Auto Zone. Here's a table of results:

Fuel Pressure: Condition
??: Turned ignition on, before cranking engine. (I forgot to note this, but I did later at the Schrader fitting on the fuel rail).
40: Started Engine from cold state. Steady reading.
40: Over the next 30 minutes and steady reading.

So, it appears that the fuel pressure at the output of my Bonnie's fuel pump is borderline. This might explain oscillating hesitation (about once a second) when my engine was calling for more fuel during a long uphill climb at 60mph and about 2200 rpm.

Next, I measured Fuel Pressure at the Schrader fitting on the fuel rail: Here's a table of results:

Fuel Pressure (PSI): Condition
28: Engine Off and cold, after sitting overnight.
38: Turned ignition on, before cranking engine.
40: Cranked/Started Engine. Took a few seconds afterwards to reach this, but engine cold.
42: Engine running for about 10 minutes, and about half-way hot.
40-44: Oscillating rapidly when engine became hot after another 5 minutes.
42: Engine Shut Off. Reached this pressure immediately.
50: 2 minutes after engine shut off. Climbed rapidly to this value, with engine still shut off.
50: 5 minutes after engine shut off.
48: 25 minutes after engine shut off.
45: 30 minutes after engine shut off.
41: 35 minutes after engine shut off.
28: 40 minutes after engine shut off. Engine warm, not hot.
40-44: 45 minutes after engine shut off. Engine warm. Started engine, and pressure oscillated rapidly.
42: Engine Shut Off. Reached this pressure immediately.

My higher-than-normal fuel pressure with the engine shut off might be caused by my fuel return line being blocked somehow, probably from deterioration on the inside of the steel return line. I'm not sure about the fuel pressure oscillation between 40-44 psi. Maybe the oscillations are caused by a faulty fuel injector?

I bought my car new in 97 (now has 166k miles), and I've never changed my gas tank, fuel pump, fuel lines, fuel rail, injectors, or anything else fuel-related, except the following items: recent air-filter, a fuel filter 2 years ago, a MAF sensor (ACDelco 2133423 [remanufactured]) last hear, and I've recently replaced my Crankshaft Position Sensor (CPS) (ACDelco 213-151), but these haven't helped for my problems above.

Any thoughts or suggestions?


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PostPosted: Sat Oct 07, 2017 1:22 pm 
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Year and Trim: 1997 Pontiac Bonneville SE
Actually, my Haynes Repair Manual might be wrong about my 97 Bonneville fuel pump pressure. It says 40-47 psi, but typical pump specs for it are 51 psi.

So, I'm not sure if my pump is borderline or way too low.

In any case, I'm going to replace my tank, pump, sending unit, etc because after 165K miles it's about time I did it anyways. It should run better afterwards.


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PostPosted: Sun Oct 08, 2017 11:28 pm 
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Fuel pressure should be 51 psi with the engine off (no vacuum being pulled on the FPR)

With the engine idling it could go down to 40 since there is vacuum on the FPR.

It should immediately go to those pressures as soon as it turns on. Based on your data the slowly lowering pressure over time probably has more to do with the change in voltage on your battery, but it does seem like there is an issue there because it should reach the correct pressure immediately.

It sounds like you might be on the right track.

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PostPosted: Mon Oct 09, 2017 5:32 pm 
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You've never changed your Fuel Pressure Regulator? Those cars were recalled years ago because the FPR would leak raw gas into the vacuum line, then when you went to start it it'd blow the plastic intake manifold up. You can call your GM dealer with your VIN# and they can see if the recall was ever performed. If it wasn't, I'd check the FPR. Did you by chance happen to notice if the fitting on the FPR was damp or smelled like gas at all?

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PostPosted: Thu Dec 28, 2017 10:18 am 
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Year and Trim: 1997 Pontiac Bonneville SE
The vacuum fitting on the FPR did not smell like gas, even after leaving the hose disconnected for a few minutes while the engine was idling. So the FPR is OK, but I'm going to change it anyways just for s**ts & giggles.

Also, while my Haynes Manual says my 97 Bonneville fuel pump pressure should be 40-47 psi, and bs009 says Fuel pressure should be 51 psi with the engine off (no vacuum being pulled on the FPR) and 40 psi with the engine idling, two fuel pumps listed on RockAuto are a Spectra-Premium SP1116 fuel pump which is spec'd as having 72.5 psi of Minimum Pressure and 101.5 psi of Maximum Pressure, while a Delphi FE0379 fuel pump is spec'd as having 51 psi of System Pressure and 65-131 psi of Relief Pressure. What's up with that? Anyone know?


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PostPosted: Thu Aug 02, 2018 2:45 pm 
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Year and Trim: 1997 Pontiac Bonneville SE
OK, I'm back again, after I installed a new Spectra-Premium GM20D Fuel Tank and SP07P1H Fuel Pump and Sender Assembly, and everything has been running OK since my last posting, about 7 months ago. I just thought I'd let you all know.


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